1. Field of the Invention
This invention generally relates to wireless local area network (wLAN) communications and, more particularly, to a system and method for synchronizing a power-save for use in 802.11e wLAN communications.
2. Description of the Related Art
A basic service set (BSS), or quality of service BSS (QBSS) typically includes an access point (AP), with a hybrid coordinator (HC), to control quality of service (QoS) traffic. When the AP has only one frame to send during a “service period” to a station (STA), or quality of service STA (QSTA), the STA may receive the frame correctly and acknowledge the frame. However, it is possible that the frame containing the STA acknowledgement may be lost, or not received by the AP. In this circumstance, the AP may retransmit the frame multiple times to insure that the frame is received by the STA. Because of the unknown possibility of these AP retransmissions, neither the AP nor the STA can determine when a service period starts. As a result, there is no means for establishing a service period synchronization that can be used by the AP, as the AP may not be sure of how long must wait before it must transmits frames to the STA. Likewise, if the AP cannot establish a service period, the STA cannot establish a block of time in which it is guaranteed not to receive a transmission. Because of this lack of guaranteed service interval, the STA cannot periodically shut down to save power. The STA cannot go into a power-save mode, even when there are long intervals between AP transmissions. Alternately, if the STA does shut down after receiving a transmission, it may lose synchronization with the AP polling schedule.
FIG. 1 is a timing diagram that illustrates the service period synchronization problem. The AP has only one frame to send. The STA is already awake, receives the frame, and sends an acknowledgement. The STA immediately goes to sleep, as the AP has indicated that there are no more frames to receive. However, the acknowledgement is lost. Because the STA is asleep, all retransmissions from the AP are lost. Shown is a second transmission from the AP that is not acknowledged by the STA. Now, the AP does not know when the STA will be awake again unless the STA initiates a frame to the AP. While the BSS can tolerate this mode of STA operation, the STA may not get access to a channel in time to support an operation, and one or more frames will be dropped at the AP.
It would be advantageous if a power-save interval could be established in a BSS that was known by both the HC and the QSTAs in communication with the HC.
It would be advantageous if battery-operated IEEE 802.11a QSTAs could be synchronized to a minimum service interval to minimize power consumption and prolong life.